This invention relates to boilers, and in particular to waste heat boilers of the type that are intended for use in marine applications.
It is known that waste heat boilers are designed to be operative to produce low cost steam from heat that would otherwise be lost. Moreover, the manner in which waste heat boilers accomplish this function, i.e., the mode of operation thereof, is well-known to those who are skilled in the art. As regards the matter of the mode of operation of waste heat boilers, one aspect thereof is of particular interest. Reference is had here to the fact that waste heat boilers operating at low pressures and low circulation ratios characteristically demonstrate extreme sensitivity to the occurrence of certain types of changes in operating conditions. More specifically, such waste heat boilers are known to be extremely sensitive to pressure or temperature changes that take place within that portion of the waste heat boiler which is known as the steam generating bank.
With respect to many of the waste heat boilers that are constructed in accordance with existing designs, it is known that relatively small changes in either operating pressure or the temperature of the entering water are capable of producing rapid changes in the proportions of water and steam that exist within the steam generating bank of the waste heat boiler. However, those changes in the proportions of water and steam that are present in the steam generating bank of the waste heat boiler can be minimized. Namely, this can be had through the proper selection of pressure control system components. Moreover, the result thereof is that the shrinkage and swelling that occurs in the amount of water which is contained in the steam drum of the waste heat boiler is attenuated.
On the other hand, changes in the temperature of the entering water produce shrinkage and swelling of the amount of water in the steam drum of the waste heat boiler. This shrinkage and swelling manifests itself in the form of large cyclic changes in water level within the steam drum of the waste heat boiler. Further, such shrinkage and swelling occasioned by changes in the temperature of the entering water cannot be attenuated simply through the selection and employment of certain particular control components.
This phenomenon of thermal shrink and swell attributable to water temperature changes is known to bear a direct relation to changes in water temperature that take place within the steam drum of the waste heat boiler. Moreover, it is further known that the changes in water temperature which take place in the steam drum of the waste heat boiler are themselves a function of the occurrence of changes in the rate at which incoming feed water flows to the steam drum.
A conventionally constructed waste heat boiler commonly includes the following components suitably interconnected in operative relation one with another: a feedwater control valve, a steam drum, a circulating pump, a steam generating bank and a centrifugal water separator. Further, in accordance with the mode of operation of such a conventionally constructed waste heat boiler the feedwater control valve is operative to modulate the flow rate to the steam drum of the incoming feedwater so as to maintain within the steam drum the desired water level, i.e., the water level that has been established based on design considerations. The steam drum on the other hand is operative as a reservoir for circulating water that is supplied by the circulating pump to the steam generating bank. Note is taken here of the fact, however, that in some installations, the circulating pump is not required if sufficient natural circulation can be obtained. The circulating water which is supplied to the steam generating bank is heated therein to saturation temperature whence steam is generated. Finally, from the steam generating bank the water-steam mixture is returned to the steam drum, and more specifically to the centrifugal water separator that is located within the steam drum. The centrifugal water separator effects the return of the water to the lower portion of the steam drum while the steam is fed to the upper portion of the steam drum.
When the conventionally constructed waste heat boiler is operating under steady state conditions, the tubing which comprises the steam generating bank thereof is divided into two different flow regimes. Namely, those tubes of the steam generating bank that receive the incoming circulating water are filled with water which is at almost constant density. That is, the incoming circulating water is being heated within the steam generating bank to saturation temperature from an initially subcooled condition.
The remainder of the tubing that comprises the steam generating bank is filled with a constant temperature water-steam mixture of gradually decreasing density. The temperature of the circulating water entering the steam generating bank of the conventionally constructed waste heat boiler determines the amount of water and the amount of steam which the steam generating bank contains. More specifically, a decrease in the temperature of the incoming circulating water causes a larger mass of water to be retained within the steam generating bank. While an increase in the temperature of the incoming circulating water causes a smaller mass of water to be retained within the steam generating bank. If the incoming circulating water is supplied at saturation temperature to the steam generating bank, no subcooled section of tubing exists therewithin. Accordingly, boiling of the incoming circulating water will take place within the initial section of tubing of the steam generating bank. Therefore, the overall water-steam mixture contained within the steam generating bank will represent the minimum mass of water obtained under steady flow conditions of a waste heat boiler constructed in accordance with conventional design.
By way of illustration of the foregoing, suppose steady conditions at 100 psig exists. Moreover, suppose subcooled circulating water is being supplied at 50.degree. F. below saturation temperature to the steam generating bank of a waste heat boiler of conventional construction. Under this set of circumstances let us assume that the steam generating bank contains six cubic feet of water and twenty cubic feet of steam. On the other hand, with no subcooling of the incoming circulating water which is supplied to the steam generating bank, the latter might contain two cubic feet of water and twenty-four cubic feet of steam. It thus can be seen that the aforedescribed change in the temperature of the incoming circulating water from a temperature of 50.degree. F. below saturation temperature to saturation temperature causes a mass of water, which is represented by the reduction of four cubic feet in water volume, to be transferred from the steam generating bank, which is a fixed volume container, to the steam drum of the waste heat boiler. Thus, since the volume of water which the steam drum contains varies with the water level therein, the effect of the aforesaid difference in the temperature of the incoming circulating water is that changes in the relative amounts of water and steam in the steam generating bank are directly reflected as changes in water level within the steam drum of the waste heat boiler.
From the preceding discussion, it should now be readily apparent that the phenomenon of cyclic thermal shrink and swell in waste heat boilers that are of conventional design and to which reference has been had previously hereinbefore is caused by the conflicting response of the system composed of the steam drum and steam generating bank to a change in the rate of flow of feedwater to the steam drum. More specifically, the aforereferenced thermal shrink and swell is occasioned by the following sequence. A decrease in water level within the steam drum causes the feedwater valve to open thereby causing an increase in the rate of flow of feedwater to the steam drum. This increase in the rate of flow of feedwater to the steam drum in turn causes a decrease in the temperature of that water which is present in the lower portion of the steam drum. Moreover, this reduction in the temperature of the water means that it is water at a reduced temperature, i.e., that present in the lower portion of the steam drum, which is fed as circulating water to the steam generating bank. The increased subcooling of the circulating water that is supplied to the steam generating bank causes a larger volume of water to be retained therewithin. Further, since this circulating water is transferred from the steam drum at a much higher rate of flow then that at which the feedwater is entering the steam drum, the water level within the steam drum falls. However, as the water level falls, additional feedwater is admitted to the steam drum causing further subcooling of the water therein. Eventually though when a temperature equilibrium of the water within the lower portion of the steam drum is established, the water level within the steam drum will begin to rise. Then as the water level increases within the steam drum, the rate of flow of the feedwater to the steam drum is reduced thereby producing an increase in the temperature of the circulating water that is being supplied to the steam generating bank. This increase in the temperature of the circulating water being supplied to the steam generating bank in turn reduces the amount of water present therewithin with the excess water being recirculated back to the steam drum and causing a rapid increase in water level within the steam drum.
It is obviously desirable that, if possible, the steam drum of the waste heat boiler be suitably constructed such that the water level therewithin is not subject to excessive excursions. That is, in the case, for example, of the hypothetical situation that was posed hereinbefore the steam drum should be capable of assimilating therewithin the additional four cubic feet of water received thereby from the steam generating bank without the water level in the steam drum being caused to vary too extensively from whatever the norm therefor is.
For those applications wherein space is not a factor, it has been known to make use of steam drums that are suitably sized so that the addition thereto, for instance, of another four cubic feet of water from the steam generating bank would not cause the water level in the steam drum to deviate from the norm beyond an acceptable amount. Namely, in the case of such applications the steam drum is made sufficiently large that another four cubic feet of water added thereto does not cause the water level to rise therewithin to an appreciable extent, i.e., to an unacceptable degree.
However, there are other applications where space is at a premium. Reference is had here in particular to marine applications in which the space allotted for locating the waste heat boiler aboard the ship is relatively limited. In these instances, the attenuation of thermal shrink and swell in the steam drum of the waste heat boiler can not be realized simply by increasing the dimensions of the waste heat boiler. In this context, the extent to which the water level of the steam drum is permitted to deviate, i.e., the magnitude of the allowed excursion thereof, from the norm therefor is commonly on the order of only .+-. two inches.
A need has thus been evidenced in the prior art for a new and improved waste heat boiler of the type that is particularly suited to be utilized in marine applications. More specifically, such a new and improved waste heat boiler has been sought which is capable of operating in such a manner that the thermal shrink and swell which takes place in the steam drum and which is occasioned by changes in water temperature is attenuated. Moreover, there is sought such a new and improved waste heat boiler for marine applications which in addition is advantageously characterized by the fact that a more efficient and effective mixing of the water within the steam drum occurs.
It is, therefore, an object of the present invention to provide a new and improved form of waste heat boiler of the type that is operative to produce low cost steam from heat that would otherwise be lost.
It is another object of the present invention to provide such a waste heat boiler which is particularly suited for use in marine applications.
It is still another object of the present invention to provide such a waste heat boiler which is effective in attenuating the thermal shrink and swell which takes place within the steam drum and which is occasioned by changes in water temperature.
A further object of the present invention is to provide such a waste heat boiler which is effective in preventing excursions of more that .+-. two inches in the water level within the steam drum.
A still further object of the present invention is to provide such a waste heat boiler that is operative to provide a more efficient and effective mixing of water within the steam drum.
Yet another object of the present invention is to provide such a waste heat boiler that is equally applicable for use in new applications as well as retrofit applications.
Yet still another object of the present invention is to provide such a waste heat boiler that is relatively inexpensive to provide, yet is reliable in operation.